Would people be interested to consider adding the CopyingMock example shown in the documentation section "coping with mutable arguments" into the mock module?

https://docs.python.org/3/library/unittest.mock-examples.html#coping-with-mutable-arguments 

It seems like a useful and simple enough feature to add.

Here's a first attempt at a patch.  

If it doesn't go into Lib finally, I'll just put up a CopyingMock on pypi instead.  But it seemed like the kind of feature which should just be in mock directly rather than 3rd party, preferably.

Guess there was no interest in this.   I've released it on pypi instead (pip install copyingmock).  

https://github.com/wimglenn/copyingmock

Thanks for releasing it on PyPI! It's probably better to keep it as an example in the documentation for now. We can reopen this if there's a demand for CopyingMock in the future.